1. Field of the Invention
The present invention generally relates to systems and methods for determining information for wafers using selective diffraction with in-series gratings.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection and metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers and/or to measure one or more characteristics of the wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection and metrology have always been important parts of fabricating semiconductor devices such as ICs. However, as the dimensions of semiconductor devices decrease, inspection and metrology become even more important to the successful manufacture of acceptable semiconductor devices because smaller defects and deviations can cause the devices to fail.
Some inspection and/or metrology systems include gratings or other dispersive elements in the detection portion of the systems that can separate the light from the wafer based on wavelength so that multiple measurements at multiple wavelengths may be performed simultaneously. The diffraction efficiency of an optical grating typically falls off to less than 10% when being operated relatively far away from its peak efficiency, which means that more than 90% of the available photons within a given system may be lost in that wavelength range. A work around for a system that requires high diffraction efficiency for both short and long wavelengths is to make two successive measurements with two independent spectrometers, which receive light from the same measurement path up to the entrance of such a dual spectrometer. A switching device such as a mirror that is flipped into and out of the beam then selects between measuring in the short wavelength spectrum or the long wavelength spectrum. A disadvantage of such a switching design is that throughput is reduced since the full spectrum measurements can only be acquired by making successive measurements.
Accordingly, it would be advantageous to develop methods and systems for determining information for wafers that do not have one or more disadvantages described above.